c-objc-method-arg-unfinished-offset

c-objc-method-arg-unfinished-offset is a variable defined in `cc-vars.el'.
Its value is

4

Documentation:
*Offset relative to bracket if first selector is on a new line.

[aaaaaaaaa
|<-x->|bbbbbbb: cccccc
ddddd: eeee];

You can customize this variable.